Many raster scanned CRT displays are now designed to be compatible with a wide range of computer systems each capable of generating one or more different raster display formats. Each raster display format is generally characterised by a different pair of line and frame scanning frequencies.
A conventional line scan circuit for driving the horizontal deflection coils for a CRT display comprises a ramp generator for producing a line scan current signal in the deflection coils in synchronisation with a raster line synchronisation (sync) signal generated by the host computer. The amplitude of the line scan signal is inversely proportional to the frequency of the line sync signal. Therefore, the width of the picture displayed on the CRT screen is inversely proportional to the frequency of the line sync signal.
The conventional CRT displays that are capable of displaying different raster display formats, a ramp regulator is connected to the ramp generator to reduce the effect of changes in line sync signal frequency on displayed picture width by varying the amplitude of the line scan signal as a function of a reference input and a feedback signal. The feedback signal is generated as a function of the line scan signal by a feedback circuit connected to the ramp generator and the regulator. The feedback circuit conventionally comprises a low pass filter with a time constant that is long enough to remove the line scan frequency component from the feedback signal. The low pass filter therefore limits the range of line scan frequencies within which the display can operate with constant picture width. Furthermore, increasing the time constant of tile filter increases the transient response time of the regulator when switching between different display formats.
In accordance with the present invention, there is now provided deflection apparatus for a raster scanned cathode ray tube display, the apparatus comprising: a ramp generator for receiving a raster synchronisation signal and for producing a raster scan signal synchronised to the raster synchronisation signal; a reference source for generating a reference input; a ramp regulator connected to the generator for varying the amplitude of the scan signal as a function of the difference between the reference input and a feedback signal; and a feedback circuit connected to the generator and to the regulator for generating the feedback signal as a function of the amplitude and frequency of the line scan signal; characterised in that the apparatus further comprises a multiplier connected to the regulator for multiplying the reference input by an integer multiple of the frequency of the raster synchronisation signal.
The present invention stems from a realisation that any scan signal frequency component in the feedback signal can be cancelled at the input to the regulator by introducing a scan signal frequency component to tile reference input. Therefore, in accordance with the present invention, displayed picture size can now be made independent of scan signal frequency. Deflection apparatus of the present invention can therefore advantageously maintain a constant picture size despite variations in scanning signal frequency without the bandwidth limitations of tile prior art.
Preferably, the multiplier comprises a switch circuit for alternating the reference input between greater and lesser reference levels in synchronisation with the raster synchronisation signal. The present invention can thus be advantageously implemented without complex low pass filter networks.
In a preferred embodiment of the present invention, the switch circuit is connected to the reference source and to the regulator for alternately connecting and disconnecting the reference source to the regulator in synchronisation with the raster synchronisation signal. The present invention may thus be advantageously implemented by a signal transistor gate in a signal path.
The multiplier may comprise a monostable circuit connected to the switch circuit for generating pulses of predetermined length for switching the switch circuit in synchronisation with the raster synchronisation signal. The monostable circuit advantageously alleviates any frequency dependency in the pulse length of the synchronisation signal.
In a preferred embodiment of the present invention to be described shortly, the generator comprises a flyback circuit; and the regulator comprises a switch circuit for varying the voltage across the flyback circuit in response to a pulse width modulated signal, and a pulse width modulator connected to the switch circuit, the multiplier, and the feedback circuit for generating the pulse width modulated signal as a function of said difference between the reference input and the feedback signal.
The integer multiple is preferably greater than one to improve noise rejection. In a preferred embodiment of the present invention, the integer multiple is, for example, set to two.
It will be appreciated that the present invention extends to a CRT display comprising such deflection apparatus.
It will also be appreciated that the present invention extends to a computer system comprising: a processor for generating raster syncronisation signals and for varying the frequency of at least one of the raster synchronisation signals to produce different display modes; and a CRT display as referred to in the preceding paragraph connected to the processor for producing a picture on the display in response to the raster synchronisation signals and for maintaining the size of the picture substantially constant between the different display modes.